The present invention relates to fasteners in general, and, more in particular to fasteners of the torque-limiting type, a process of setting such a fastener, a resulting joint, and a driver used in the setting.
In a standard threaded fastener system of a male threaded fastener and a female threaded freshener, the female fastener has internal threads that thread onto external threads of the male fastener. Wrenching surfaces of both fasteners accept tools that tighten them and clamp one or more workpieces together between them, oftentimes with washers interposed in between. The combination of the fasteners and the workpieces are known as a "joint." Male threaded fasteners are variously known as "screws," "bolts," or "pins;" female threaded fasteners are variously known as "nuts" or "collars;" workpieces are sometimes called "sheets" or "structural elements."
Fasteners bear loads along their axes, tensile loads, and radially of their axes, shear loads. Tensile loading always exists because of the clamping force applied by the pin and the collar to the sheets; this load is known as "clamp-up" or "pre-load." When fasteners join two or more sheets and the sheets are loaded in their planes, one sheet may tend to slide over the other; when this loading of the sheets occurs, it is resisted by the fasteners, and the sheets load the fasteners in shear. Shear loads are transverse to the axes of the fasteners and transverse to the tension load. Cyclic loading of a fastener can produce fatigue failure. In aerospace applications shear failure is usually most critical in fatigue.
Adequate clamp-up or preload is absolutely necessary for a satisfactory joint. A fastener adequately loaded by the reaction to the clamp-up load resists fatigue failure. Preload also helps the structural elements to resist fatigue failure. Accordingly, it is desirable to know the clamp-up load the fastener applies to a structure to be sure that a joint has adequate fatigue strength. Adequate clamp-up also avoids sheet slippage and fretting and insures against load shifting and joint failure.
Clamp-up load correlates to the resistance of a collar to further threading onto a pin and against a workpiece by the application of torque to the collar. As clamp-up force increases, the resistance to further threading increases, and the torque required to turn the collar increases.
This fact has been used in fasteners to develop a predetermined clamp-up load by termination of tightening through failure of a wrenching section on the collar. U.S. Pat. Nos. 2,940,495 to G. S. Wing and 4,260,005 to Edgar Stencel describe two types of such fasteners.
The Wing patent describes a collar extensively used in the aerospace industry. It has a wrenching section connected to an internally threaded section by a frangible break-neck collar. The collar breaks upon the application of a predetermined torque that corresponds to a desired clamp-up load. An acircular portion of the threaded section provides a thread lock by pressing tightly against the threads of the cooperating pin. A problem with this type of fastener is what it generates a waste piece: the wrenching section. The waste piece must be removed from the environment where the fastener is set. This type of fastener is also comparatively expensive because it requires a considerable amount of machining to make it and the frangible break-neck must be held to very close tolerances to provide close tolerances in break-off torques.
The stencil patent describes a collar that has a plurality of circumferentially spaced lobes on its axial outside that serve as wrenching surfaces and in torque limitation. A wrenching tool, say a triangular shaped socket, has flats that engage flanks of the lobes and turn the collar with respect to the pin. Upon reaching a predetermined clamp-up load, the lobes fail in radial compression and merge into the body of the collar, and wrenching and tightening stops because the lobes no longer provide purchase for the setting tool. The Stencel collar produces a thread lock by a deformation of collar material radially inward of the lobes against the threads of a cooperating pin when the lobes fail.
Impact wrenches used in setting fasteners do so rapidly. The failures of the break-neck of the Wing fastener and of the lobes of the Stencel fastener occur over very few degrees of rotation, and, when an impact wrench is used, occur very rapidly. The rapid application of setting torques to a collar can result in loss of some desired pre-load through relaxation of the sheets; relaxation results from the continued deformation of the sheets after the initial loading. Such deformation reduces the load per unit area and absolute loading because material moves away from the clamped zone. When the break-neck or the lobes fail, they fail at a torque corresponding to a desired pre-load. But the loaded sheets can relax and some of the pre-load will be lost. This relaxation is a time-dependent phenomenon, and with slower development of pre-load, relaxation and loss of pre-load will be less.
It may also be desireable to be able to change the pre-load even with the same collar. For example, when the sheets are not as strong in compression as some other sheets, it may be necessary to lower the compressive load on them.
In some applications secondary wrenching is desired in order to increase pre-load above design pre-load or to compensate for relaxation. Secondary wrenching is impossible in the standard configurations of the Wing and Stencel collars. These collars are also difficult to remove after they have been set because of the absence of wrenching sections.
An important requirement of an aerospace fastener is a known and repeatable clamp-up load. The clamp-up load correlates directly with the torque that sets the fastener. Nonetheless, a lot of the setting torque in a typical fastener system is not used in developing clamp-up, but instead is used in overcoming friction. The reduction of parasitic friction has the advantages of reducing the driving load, reducing the requirements of the setting tools, and increasing the accuracy of the clamp-up load.
In a fastener system where pre-load is determined by the failure of some external wrenching means, such as the lobes on the Stencil collar, swelling of the collar because of radial loads applied to it can adversely affect the pre-load.
Many fastener systems have a thread lock to keep the collars from loosening on the pins. A form of thread lock uses deformed thread of the collar to increase friction between the collar and the pin threads in a localized area. Substantial hoop stress on the collar imparted through the threads of the pin can reduce or eliminate the effect of the thread lock because the hoop stress overcomes the deformation in the collar and plastically deforms the thread lock so that it loses its ability to perform its function.